This section provides background information that may or may not include prior art.
Isocyanate oligomers, such as uretdiones, biurets, and isocyanurates, have been used as crosslinkers in curable coating compositions. For example, such isocyanate oligomers, blocked or unblocked, have been used to crosslink active hydrogen-containing binder systems. The isocyanate groups that are not part of the isocyanurate, biuret, or uretdione ring react with the active hydrogen groups of the binder if unblocked, or, if blocked, undergo that reaction under unblocking conditions. Usually, unblocking occurs at an elevated temperature. Catalysts may be added to facilitate the reaction of the free or unblocked isocyanate groups with the active hydrogen groups of the binder.
Spyrou et al. of Degussa AG have described polyurethane compositions containing uretdione groups in U.S. Pat. Nos. 7,307,135; 7,300,997; and 6,914,115 and in U.S. Patent Application Publications 2008/0269415; 2008/0139753; 2008/0097025; 2007/0266897; 2005/0239956; 2005/0239992; 2005/0119437; 2005/0096450; 2005/0003206; and 2004/0219367. The coating binders contain a uretdione hardener based on an aliphatic polyisocyanate having very low free isocyanate content and a hydroxyl-containing compound, along with a catalyst to accelerate cleavage of the uretdione groups. The isocyanate groups generated on cleaving the uretdione cure the hydroxyl-containing compound. The uretdione hardener is made by reacting a polyisocyanate in the presence of a dimerization catalyst to a desired conversion, e.g. isocyanate (NCO) content of less than 5% by weight, at which point a catalyst poison is added. The hydroxyl-containing compound that is cured may be a polyester, polyether, polyacrylate, polyurethane, or polycarbonate polymer. Mazanek et al., U.S. Patent Application Publication 2007/0032594 describes an aqueous dispersion of a self-crosslinking polyurethane containing uretdione groups, carboxylic acid or carboxylate groups, and amine or hydroxyl groups.
Certain patents have described coating compositions that cure through more than one kind of reaction, for example Meisenburg et al., U.S. Pat. No. 7,446,142; Neimeier et al., U.S. Pat. No. 7,416,781; Baumgart et al., U.S. Pat. No. 7,129,280; Ramesh, U.S. Pat. No. 6,569,956; December et al., U.S. Pat. No. 6,048,443; and Ohrbom et al., U.S. Pat. No. 5,945,499. Certain patents have described coatings additives covalently bound to a binder polymer, for example Boisseau et al., U.S. Pat. No. 6,391,390; St. Aubin et al., U.S. Pat. No. 6,180,240; Ohrbom et al., U.S. Pat. No. 6,166,148; Ohrbom et al, U.S. Pat. No. 6,037,441; Oberg et al., U.S. Pat. No. 5,872,165; and Johnson, U.S. Pat. No. 4,504,628. These patents and the described uretdione patents do not, however, suggest or presage the unexpected benefits and advantages available from the compositions and processes now disclosed and described.